This invention relates to a fluoropolymeric composite material. More particularly, this invention relates to a fluoropolymeric composite material which is particularly well suited for use as a bonding ply in a multiplayer circuit board and in other applications requiring the ability to flow as well as good thermal, mechanical and electrical properties.
U.S. Pat. No. 4,849,284 which is assigned to the assignee hereof and incorporated herein by reference describes a ceramic filled fluoropolymer-based electrical substrate material sold by Rogers Corporation under the trademark RO-2800. This electrical substrate material preferably comprises polytetrafluoroethylene filled with silica along with a small amount of microfiberglass. In an important feature of this material, the ceramic filler (silica) is coated with a silane coating material which renders the surface of the ceramic hydrophobic and provides improved tensile strength, peel strength and dimensional stability. The composite material of U.S. Pat. No. 4,849,284 discloses a volume % filler fraction (on a void free basis) of at least 50 for use as a circuit substrate or a bonding ply.
The ceramic filled fluoropolymer-based electrical substrate material of U.S. Pat. No. 4,849,284 is well suited for forming rigid printed wiring board substrate materials and exhibits improved electrical performance over other printed wiring board materials. Also, the low coefficients of thermal expansion and compliant nature of this electrical substrate material results in improved surface mount reliability and plated through-hole reliability. As is known, individual sheets of this electrical substrate material may be stacked to form a multilayer circuit board. In fact, thin film formulations of the material disclosed in U.S. Pat. No. 4,849,284 (and sold by Rogers Corporation under the trademark RO-2810) may be used as a bonding ply to bond together a plurality of stacked substrate layers so as to form the multilayer circuit broad.
It will be appreciated that high volume fractions (greater than 55 volume %) of ceramic filler will significantly and adversely effect the rheology (e.g. flow) of the ceramic filled fluoropolymer composite. This is particularly important when the composite is used as a bonding film or in filling openings in previously rigid structures. While ceramic filler volume fractions of 50-55 % provide significantly improved rheological properties relative to higher filler fractions, there is a perceived need to provide even better flow properties to the fluoropolymeric composite without appreciably altering the excellent thermal, mechanical and electrical properties.
In U.S. Ser. No. 483,501 filed Feb. 21, 1990, now U.S. Pat. No. 5,024,871. (which is assigned to the assignee hereof and fully incorporated herein by reference), it had been discovered that the silane coating on the ceramic filler of the electrical substrate material disclosed in U.S. Pat. No. 4,849,284 may be replaced with zirconate coupling agents and/or titanate coupling agents and the resultant composite material will still retain the excellent thermal, mechanical and electrical properties as that described in U.S. Pat. No. 4,849,284.